Several techniques are known for applying electroscopic toner to latent electrostatic image bearing record members so that toner will adhere to the record member in desired areas to develop the image. One technique is by cascading the particulate toner in dry form onto the surface of the record member, and removing the excess toner either by allowing it to slide off the record member by the force of gravity, or by blowing the excess toner with a fan. Another technique is to entrain the toner particles in a finely divided magnetic powder and to use a magnetic brush for distributing the toner over the surface of the record member. A third technique for distributing the toner is to entrain the toner in a dielectric liquid which is then brought in contact with the surface bearing the latent image. The developer apparatus of the present invention utilizes the latter technique.
A significant problem encountered in developer apparatus that uses liquid entrained toner is commonly known as "boundary layer depletion." This may occur even after a very short period of use, since the concentration of toner in the liquid near where the liquid is brought into contact with the recording member may lower very rapidly when developing a "dark area" of the latent image.
Other than the total immersion of the recording member in the liquid entrained toner for the purpose of development, which is not very effective with flat record members, there has existed the problem of supplying an adequate concentration of toner to ensure complete development. One of the earliest attempts to solve this problem was to use a roller dipped into a liquid toner bath The roller, which was also conducting, was then rotated to provide a constant changing and replenishing action to the liquid entrained toner at the processing nip. Such an apparatus is illustrated in U.S. Pat. No. 3,367,791.
As processing speeds increased, equipment modifications were required to keep pace with the increased through-put. It was found that as the recording member was passed over a rotating developing electrode, the developing zone was relatively small and that effective development was limited by the volume of liquid dispersed toner that the rotating electrode could deliver to the processing nip.
U.S. Pat. No. 3,561,400 relates to an improved apparatus that attempted to solve many of the problems mentioned above. Generally, the apparatus comprises a shroud that surrounds a cylindrical roller. The shroud has an opening therein which permits a portion of the roller to project beyond the shroud. A pump provides a supply of liquid entrained toner into the space between the roller and the shroud. The roller is rotated to carry a film of the developer liquid over the projecting portion of the roller to contact the record bearing members.
An attempt to increase the through-put of such an apparatus would necessitate a drastic increase in the diameter of the rotating electrode to obtain a proper development zone as well as an adequate electrical field in the development zone.